A hearing impaired person typically suffers from a loss of hearing sensitivity, this loss dependent upon both the frequency and the audible level of the sound in question. Thus a hearing impaired person may be able to hear certain frequencies (e.g., low frequencies) as well as a non-hearing impaired person, but unable to hear sounds with the same sensitivity as the non-hearing impaired person at other frequencies (e.g., high frequencies). Similarly, the hearing impaired person may be able to hear loud sounds as well as the non-hearing impaired person, but unable to hear soft sounds with the same sensitivity as the non-hearing impaired person. Thus, in the latter situation, the hearing impaired person suffers from a loss of dynamic range.
A variety of analog and digital hearing aids have been designed to mitigate the above-identified hearing deficiencies. For example, spectral enhancement techniques increase the dynamic range contrast in the speech spectrum. The objective of spectral enhancement is to improve the ease of listening and possibly the level of speech intelligibility. With respect to dynamic range loss, typically a compressor is used to compress the dynamic range of the input sound so that it more closely matches the dynamic range of the intended user. The ratio of the input dynamic range to the dynamic range output by the compressor is referred to as the compression ratio. Generally the compression ratio required by a user is not constant over the entire input power range.